XIncoherentInstances doesn't work
I've been messing around with the syntactic library (version 2.0 on github), and I have a small example that causes GHC(i) 7.6.2 to complain about IncoherentInstances .
{-# LANGUAGE FlexibleInstances,
TypeFamilies,
TypeOperators,
OverlappingInstances,
UndecidableInstances,
GADTs,
FlexibleContexts,
IncoherentInstances #-}
import Data.Syntactic
import Data.Syntactic.Functional
type family FunToAST (dom :: * -> *) f
type instance FunToAST dom (Full a) = ASTF dom a
type instance FunToAST dom (a :-> fi) = ASTF dom a -> FunToAST dom fi
data Let a where
Let :: Let (a :-> (a -> b) :-> Full b)
share :: (Let :<: dom,
ApplySym sig fi dom,
SyntacticN (Denotation sig) fi,
fi ~ FunToAST dom sig)
=> Denotation sig
share = sugarSym Let
Fine. I'm not asking why I need IncoherentInstances . The problem is, when I include that extension, I get the exact same error message:
Overlapping instances for SyntacticN
(Denotation sig)
(ASTF sup0 a0 -> ASTF sup0 (a0 -> b0) ->
ASTF sup0 b0)
arising from a use of `sugarSym'
Matching instances:
instance [overlap ok] (Syntactic f, Domain f ~ sym,
fi ~ AST sym (Full (Internal f))) =>
SyntacticN f fi
-- Defined in `Data.Syntactic.Sugar'
instance [overlap ok] (Syntactic a, Domain a ~ sym,
ia ~ Internal a, SyntacticN f fi) =>
SyntacticN (a -> f) (AST sym (Full ia) -> fi)
-- Defined in `Data.Syntactic.Sugar'
(The choice depends on the instantiation of `sig, sup0, a0, b0'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the expression: sugarSym Let
In an equation for `share2': share2 = sugarSym Let
I doubt this problem is actually related to the library, but I don't know enough about IncoherentInstances to write my own example.
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